1. Field of the Invention
The invention is related to nuclear reactor coolant systems and particularly to the supply of emergency coolant to the nuclear reactor core during a loss of coolant accident.
2. General Background
In the nuclear power industry, nuclear reactors must be designed so as to withstand a variety of postulated accidents. Probably the most traumatic, if not most important, potential accident to be avoided is a loss of coolant accident (LOCA) involving the nuclear reactor. This occurs when there is a breach in the cooling system causing a drop in pressure along with a loss of coolant. As can be imagined, without any coolant in the reactor core, the core will overheat, potentially causing severe damage not only to the structure but also to the surrounding environment.
The principal method of mitigating such an accident is to flood the reactor with coolant. This must occur as soon as possible after the cooling system loses pressure in order to absorb the heat that builds up in the core during the LOCA. This usually involves adjacent storage tanks of emergency coolant that are coupled to the reactor piping. In some cases these tanks are under pressure and in others, pumps are provided to deliver the coolant to the reactor. This emergency coolant is injected into the reactor piping in any of several locations such as hot leg piping, cold leg piping, through the wall near the top of the reactor vessel, or by piping arrangements to the plenum area above the reactor core. However, one major drawback of tying into the reactor piping is the possibility that the break will prevent the fluid from reaching the reactor core. Another drawback of existing designs is that early in the LOCA, flashing in the lower vessel prevents normal coolant already in the vessel from reaching the core. A third drawback is that steam produced in the core acts to prevent additional emergency coolant from reaching the core.
Typically, during a LOCA, the core flood tanks of a nuclear reactor are designed to deliver emergency coolant to the reactor vessel. However, for nuclear power plants currently in operation, this occurs well in time into the LOCA, thus "flashing" throughout the reactor vessel will occur prior to the introduction of the emergency coolant. "Flashing" is a term generally used to describe the vaporization of hot pressurized liquid when it is rapidly depressurized. This occurs in a nuclear reactor when as the result of the rupture of the normal coolant piping, normal cooling fluid is exposed to significantly reduced pressure as normal coolant is expelled out the rupture. Depending on temperature and pressure, such flashing and loss of normal coolant fluid can occur within a matter of seconds.
It is important to deliver emergency coolant to the reactor as quickly as possible to minimize any core temperature increases that occurs whenever flashing and boiling in the core occurs. It is vitally important to supply emergency coolant to the core inlet after such rapid de-pressurization so as to continue coolant flow to the core thus minimizing or eliminating core temperature increases. It is also the intent of emergency coolant to prevent or minimize the occurrence of flashing from depleting the normal coolant fluid already residing in the reactor vessel lower plenum and downcomer.
In view of the present state of the art, there is a need for an improved means for emergency core cooling of a nuclear reactor during a LOCA.